Indeed - I bought an old set that didn't seem that dynamic; refreshed the EHT units and away they go. The diodes break down over time - I'd go as far as suggesting if they haven't been done then do them anyhow.
Fairly straightforward DIY exercise assuming you have the wax encased designs - I did a full write up on PFM when I did mine a couple of years back. You just need to cook them for a while to melt the wax. Change the caps at the same time - even though they are probably OK, better to be safe given the low additional cost/time to replace them too.
Oh, it brings them to life too
Richard